Presently there are two methods of providing well distributed illumination from light sources. In the first, a large enough number of light sources is employed to ensure that the resulting illumination has a sufficient degree of uniformity for the task at hand. This method is well known to those skilled in the art of illuminating engineering.
In the second method, a smaller number of light sources is utilized, and uniformity of illumination is achieved by distributing the light energy by means of light guides. This technique is exemplified by U.S. Pat. No. 4,750,798, which issued to L. Whitehead on June 14, 1988, which describes an illumination system in which the light guide is a prism light guide, and U.S. Pat. No. 4,105,293, which issued to Aizenberg et al. on Aug. 8, 1978, in which the light guide is a metallic reflector light guide.
The first method produces an irritating strobe light effect whenever a viewer travels relative to the light system such as in a subway or on a bridge.
The second method of providing distributed illumination is advantageous due to the economies of scale of utilizing a smaller number of more intense light sources. However, this reduction in the number of light sources only serves to exacerbate the problem which both techniques have, namely that failure of light sources can result in dangerously low levels of illumination.
Such failures can be particularly hazardous in situations where the light from sources adjacent to those which have failed cannot propagate very readily, and where there is a threat to life safety in the event that illumination is insufficient. A good example of such a situation is illumination in subway tunnels, where if a train becomes incapable of moving, passengers must walk out of the tunnel under potentially hazardous conditions, and where the narrow dimensions of the tunnel greatly reduce the extent to which light can propagate.
There are already known solutions to this problem of light loss from failure of sources. For example, it is possible at extra cost to increase the number of light fixtures, and/or the number of lamps per light fixture so that individual lamp failures do not cause too great a reduction in light. However, this is expensive, and does not eliminate the possibility of failure of an entire group of lamps because of a wiring problem. Another solution is to provide independent battery-operated electric lamps which turn on in the event that light from the main system fails, however this is generally a prohibitively expensive solution.